High-yield and ultra-high yield wood pulps undergo rapid light-induced discoloration, particularly when they are exposed to near ultraviolet light (wave lengths 300-400 nm) in indoor fluorescent light and daylight. This characteristic restricts their use to short-life, low-value paper products. High-yield and ultra-high yield wood pulps can be bleached to a high level of whiteness. If this whiteness could be stabilized against discoloration, these bleached high-yield pulps could displace significant amounts of more expensive fully-bleached, low-yield chemical pulps.
This discoloration is ascribed to the substantial lignin content of high-yield pulps totaling about 20-45% by mass. Phenoxy radicals are the key intermediates in the reaction mechanism. Several light-induced reactions have been proposed to account for their formation such as abstraction of a hydrogen atom from phenolic groups, cleavage of the aryl ether bond of phenacyl aryl ether groups, or breakdown of ketyl radicals formed from saturated aryl-glycerol .beta.-aryl ether structures in lignin. The phenoxy radicals are oxidized by other oxygen-centered radicals (alkoxy, peroxy, hydroxy and perhydroxy) to form yellow chromophores. (C. Heitner in "Photochemistry of Lignocellulosic Materials", C. Heitner, J. C. Scaiano, eds,: ACS Sym. Ser. 531, 1-25 (1993).)
I. E. Arakin et al., Khymiya drevesiny (Chemistry of Wood), 1982, No. 2, 114 and A. D. Sergeev et al., ibid, 1984, No. 5, 20 disclosed that the use of iminoxyl radicals such as TEMPO (1-oxyl-2,2,6,6-tetramethylpiperidine) is useful in the delignification of wood using the one-stage oxygen-soda (alkaline) process, but made no mention or suggestion of any activity provided by TEMPO on preventing light-induced discoloration of paper or pulp made from such treated wood.
EP 717,143 and WO 97/36041 describe a multicomponent system for changing, reducing or bleaching lignin and lignin-containing materials which comprise a oxidation catalyst, and a N-hydroxyl mediator compound such as a N-hydroxyphthalimide or a dialkylhydroxylamine. These references are aimed at the delignification of wood. There is no mention or suggestion of any activity provided by the N-hydroxyl compounds in preventing the light-induced discoloration of paper or pulp made from such treated wood.
V. I. Khodyrev et al., Vysokomol soyed, A29, No. 3, 616 (1987) [Polymer Sci. U.S.S.R., 29, No. 3, 688 (1987)] show that the photoinitiated oxidation by oxygen causes weathering of cellulosic textile materials such as flax or cotton. The UV absorbers offer no protection, and are actually detrimental. The authors noted that the stable nitroxyl radical 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine interacts with alkyl radicals in the cellulose to afford its salubrious stabilizing activity. There is no suggestion by the authors that this stabilizing activity could be applied successfully in lignin-containing pulp and/or paper made therefrom.
M-K. Syker et al., J. Assn. Paper Pulp Tech, 29, 135 (1990) show that selected metal salts such as magnesium sulfate and lower alkanoic acids inhibit color reversion in bleached pulp.
P. Fornier de Violet et al., Cellulose Chem. Tech., 24, 225 (1990) show that the use of UV absorbers and hydrogen donor agents such as thiols, ascorbic acid, etc. help prevent the photoinduced discoloration of hydrogen peroxide bleached wood pulp, but that chain breakers such as hindered phenols and hindered amines (having &gt;N--H or &gt;N--CH.sub.2 -- moieties) had no or even a detrimental effect on preventing photoinduced discoloration.
R. Agnemo et al., Holzforschung (1991), 45 (Suppl), 101, confirmed that free hydroxyl radicals plus lignin lead to undesirable photoyellowing in pulp and paper.
S. Omori et al., J. Assn. Paper Pulp Tech, 48, 1388 (1993) describes the effect of antioxidants and UV absorbers on light reversion and concludes that the combination of an antioxidant and UV absorber prevents color reversion and has a synergistic effect in that activity.
M. Paulsson et al., Nordic Pulp Pap. Res. J., (1995), 10 (1), 62-67, show that efficient photostabilization of unbleached paper or hydrogen peroxide bleached TMP pulp can be achieved by acetylation.
There have been a number of different approaches proposed to inhibiting the yellowing of mechanical pulps. These include: radical scavengers and antioxidants; UV screens; elimination of chromophores after their formation; chemical modification of lignin by alkylation or acetylation; polymeric inhibitors; and two types of coadditives used in combination. Z-H. Wu et al., Holzforschung, 48, (1994), 400 discuss the use of radical scavengers like phenyl-N-tert-butylnitrone to reduce the formation of chromophores during mechanical pulping and give a more light-stable pulp.
C. Heitner, "Chemistry of Brightness Reversion and It Control, Chapter 5", in Pulp Bleaching-Principles and Practice, C. W. Dence, D. W. Reeve, eds., TAPPI Press, Atlanta, 1996, pp 183-211, summarizes the state of the art in the thermal and light-induced yellowing of lignin-containing pulps such as thermomechanical (TMP) and chemithermomechical (CTMP) pulps, showing the seriousness of these undesirable effects discusses generally the then current prior art methods used to attack this problem. These include bleaching, the use of phosphites, UV absorbers, polyalkylene glycols and free radical scavengers such as ascorbic acid, thiols, thioethers, dienes and aliphatic aldehydes and chelating agents such as ethylene-diaminetetraacetic acid (EDTA). The author concluded that, although much progress had been made, much still remains to be done before a successful and practical solution to this loss of brightness and undesirable yellowing of lignin-containing pulp and/or paper is found.
The instant invention described in detail below provides such a solution where the use of selected hindered amine nitroxides, hindered amine hydroxylamines or their salts in combination with selected UV absorbers and metal chelating agents is seen to prevent loss of brightness and to enhance resistance to yellowing in pulp or paper still containing lignin.